For some time it has been desired to decrease recording medium thickness and improve the coercive force to operate at high density recording levels. Conventionally, .gamma.-Fe.sub.2 O.sub.3 fine particles are generally coated with binder on a substrate to form a .gamma.-Fe.sub.2 O.sub.3 coated medium, thereafter the coated .gamma.-Fe.sub.2 O.sub.3 is hardened to form a .gamma.-Fe.sub.2 O.sub.3 disk medium. Alternatively, a .gamma.-Fe.sub.2 O.sub.3 film is prepared by reactive sputtering from an iron target onto the substrate and the resultant .alpha.-Fe.sub.2 O.sub.3 film is reduced by heating in H.sub.2 gas to form a Fe.sub.3 O.sub.4 film and the resultant Fe.sub.3 O.sub.4 film is oxidized by heating in air to form the desired .gamma.-Fe.sub.2 O.sub.3 film. Thus resultant .gamma.-Fe.sub.2 O.sub.3 films have been developed as magnetic disk media (J. Appl. phys. VOL. 53 No. 3 1982. page 2556 to 2560). To the .gamma.-Fe.sub.2 O.sub.3 film Co is added to increase the coercive force (Hc) (IEEE, Trans. Mag. VOL.MAG-15 1979 page 1549 to 1551).
Cu may also be added to .gamma.-Fe.sub.2 O.sub.3 film to extend the lower limit of reduction temperature. As a substrate for the magnetic disk used in this method, a Al-alloy plate polished and coated with an anodized layer (alumite) may be used. When this substrate is heated over 320.degree. C., the surface of Al-substrate is caused to roughen and the coated Al.sub.2 O.sub.3 layer is cracked. Therefore, the process of reduction from .alpha.-Fe.sub.2 O.sub.3 to Fe.sub.3 O.sub.4 is a critical process in the fabrication of .gamma.Fe.sub.2 O.sub.3 film. It is necessary that the lower limit of reduction temperature is extended toward the lower temperature side in order to fabricate uniform .gamma.-Fe.sub.2 O.sub.3 film medium having excellent magnetic and mechanical properties on the substrate.
To .gamma.-Fe.sub.2 O.sub.3 film Ti may be added to improve the squareness of hysteresis loop. .gamma.-Fe.sub.2 O.sub.3 films to which have been added Co, Ti, and Cu thus show improvement of the coercive force and the effect of extending the lower limit of reduction temperature. However, it is known that .gamma.-Fe.sub.2 O.sub.3 film having the above-mentioned metals have a lower saturation magnetization (4.pi. Ms). It is believed that these metal ions cause a lowering of the magnetic moment, these metal ions also influence the amorphous non-magnetic phase and the lattice defect obtained in sputtering film. Additionally the resultant films are porous.
Co as an additive is effective to increase the coercive force in fabrication of .gamma.-Fe.sub.2 O.sub.3 film but causes reduction of the saturation magnetization and causes further deterioration of the squareness of hysteresis loop. Therefore, a recording medium having higher saturation magnetization is required in the fabrication of .gamma.-Fe.sub.2 O.sub.3 film disk.
One of the objects of the application of .gamma.-Fe.sub.2 O.sub.3 film medium is as a magnetic recording disk. The maximum value (Hs) of the horizontal component produced from a magnetic disk head can be calculated according to Karlqvist's equation (M. MATSUMOTO "Magnetic recording" Kyoritsu Shuppan Kabushiki Kaisha page 21 (1977)). EQU Hs-4 Ms cot.sup.-1 (2y/g)
herein
Ms: Saturation magnetization of head material PA1 y: head-medium spacing PA1 g: head gap length
When using ferrite, as many head materials have, a saturation magnetization 400 Gauss, head gap of 0.8 .mu.m and head medium spacing 0.2 .mu.m and medium thickness 0.1 .mu.m in magnetic recording, the horizontal component (Hs) reached can be calculated as about 1500 Oe. If Hx of the hysteresis loop of the magnetic film shown in FIG. 1 is more than 1500 Oe, this medium does not saturate under the above mentioned recording conditions, resulting in the so-called unsaturation recording. This situation causes poor overwrite and erase characteristics.
There is a relation in .gamma.-Fe.sub.2 O.sub.3 film, Hx=.alpha.Hc, herein .alpha. is 1.8 to 2.0 in .gamma.-Fe.sub.2 O.sub.3 film. When the coercive force has a value more than about 800 Oe in the recording condition, Hx.gtoreq.1500 Oe. This value becomes larger than above-mentioned Hs value. When the coercive force increases to realize high recording density, it is necessary to maintain .alpha. as low as possible. Ideally .alpha.=1. On the other hand, coercive squareness S,.sup.*, showing the slope at point of coercive force of hysteresis loop, has the relationship S*=Hr/Hc. S* value influences the recording density in saturation magnetization recording.
When the magnetic field distribution caused from the head is constant and S* becomes larger, recording density increases due to the narrowing width, a, in the magnetization transition region in the medium. To .gamma.-Fe.sub.2 O.sub.3 film usually are added several atom % of Ti and Cu to improve S* of disk media. .gamma.-Fe.sub.2 O.sub.3 films having S* =0.77 is used in practice as magnetic recording disk media.
The relation between width a of the transition region and recording medium characteristics such as film thickness d, residual magnetization Mr, coercive force Hc, and S.sup.* have been investigated and analyzed by Talke et al (IBM. J. Res. Develop 19 page 591 to 596 (1975)). The relation between the width a of transition region and recording density D.sub.50 have been investigated by Comstock (IBM. J. Res. Develop 18 page 556 to 562 (1974)), herein recording density D.sub.50 is the recording density where the output attenuated to half of the isolated output.
Based on the above-mentioned equation, the dependence of recording density D.sub.50 on Hc or S* can be calculated. When S* increases about 0.1, recording density D.sub.50 increases about 100 FRPM (Flux Reversal per millimeter). When Hc increases 100 Oe, D.sub.50 increases about 100 FRPM. This is calculated given 0.12 .mu.m in thickness d, 240 Gauss in residual magnetization, 0.15 .mu.m in head gap length, 0.1 .mu.m in head flying height, 700 to 1000 Oe in Hc and 0.60 to 0.95 in S* . The improvement D.sub.50 means the increase of read back output in high recording density. If the noise voltage produced from the disk medium is kept constant, it is obvious that improvement of the signal to noise ratio is carried in disk medium.